Mr N

Everything I want to remember

Allow remote JMX management for Tomcat

Posted by on 19 Jan 2009 in Techie
In order to switch on JMX to monitor a Tomcat instance you must firstly switch it on and then tell it what port to listen on. There is also optional support for user names and passwords. JMX helps with remote management and as a consequence will generally let you do some pretty evasive stuff with the internals of your application. So if it is in a production environment passwords might be a good idea! During Tomcat’s start up sequence it calls the file TOMCAT_HOME/bin/setenv.bat (or TOMCAT_HOME/bin/setenv.sh depending on your environment) which sets up any extra environment variables or Java VM command line options. In some of the Tomcat distributions this file does not exist, so you must create it yourself. Tomcat’s servlet container is called Catalina so inside TOMCAT_HOME/bin/setenv.bat we must set our JMX options in the environment variable CATALINA_OPTS. All the arguments must go on the same line.
set CATALINA_OPTS=-Dcom.sun.management.jmxremote ^ -Dcom.sun.management.jmxremote.port=8181 ^ -Dcom.sun.management.jmxremote.authenticate=false ^ -Dcom.sun.management.jmxremote.ssl=false
There is extra information on the Sun site related to user name and password authentication, but this was not set by default. The default was to have no authentication. After makeing the changes, restart Tomcat.

Switch on Tomcat JMX under Eclipse

Posted by on 18 Jan 2009 in Fun
In order to get JMX information out of a running instance of Tomcat under Eclipse, you must add the Sun’s JMX Remote Management debug option to the start-up command of Tomcat.
  1. Goto the Servers pane at the bottom of the screen. (Or Window->Show View->Server to open the Servers View.)
  2. Open Tomcat Overview configuration window by double clicking on Tomcat v6.0 Server at localhost.
  3. Under General Information click the Open launch configuration
  4. Select the Arguments tabs of the Edit launch configuration properties
  5. In VM arguments add
    -Dcom.sun.management.jmxremote
  6. Click Ok and close the Tomcat Overview configuration window.
I say this is under Eclipse but the same action should be taken when running Tomcat outside of Eclipse.

Redirecting web pages

Posted by on 17 Jan 2009 in Techie
If you move a web page to a new location on your server you don’t want to create broken links. To make sure your web site still fits together you can create page place holders that redirect visitors to a different location. This will give you time to change the related pages to point to the new location. You must also spare a thought for all those pages on other web sites that point to your old page. There are 2 ways this can be achieved. HTML has the advantage of allowing you to include an accompanying page to display first and then after a given time period redirecting the visitor to the new location. You may want to explain to your visitor why you are redirecting them before you send them on their way. 
<HTML><HEAD>
<META HTTP-EQUIV="refresh" CONTENT="3;URL=http://www.bigsoft.co.uk/">
</HEAD<BODY>Our name has changed</BODY></HTML>
The CONTENT contains 2 parts. The time in seconds is followed by a URL to redirect them to. The second method is via some kind of programming language like PHP. 
<?php
header("Location: http://www.bigsoft.co.uk/");
?>
This method works slightly differently, because it injects a Location directive into the HTTP response, where as the META tag is interpreted by the browser to do the redirect.

Prevent Tomcat redeploying when running Eclipse in debug mode

Posted by on 05 Jan 2009 in Techie
Been spending a lot of time developing web applications under Eclipse, using Tomcat as the servlet container. The most valuable aspect of using Tomcat and Eclipse together is being able to run your web application in Tomcat debug mode. The coolest thing about it is that you can fix problems while the application container is running. When Eclipse detects that you have saved a file, it recompiles the method or class that has changed and injects it back into the application drops a stack frame and places your run line pointer back to the top of the new code. How cool is that? Tomcat has a change detection system that notices that one of the classes has changed and tries to redeploy the application. Normally under a live environment this is a good thing because it is a nice easy way of installing new software with the minimum of down time. Under development, however, this is a real pain. Not only does the application run at half speed (because it is in debug mode), often most of the modules can’t cope [yet ;)] with the redeploy signal they get and become confused - which is never a good thing! To tell Tomcat not to redeploy when it notices a change is quite simple, once you know how. Place this file in WebContent/META-INF/context.xml: 
<?xml version="1.0" encoding="UTF-8"?>
<Context path="/context" debug="1" reloadable="false">
  <Parameter name="runtimeEnvironment" value="DEVEL" override="false"/>
  <!-- other directives -->
</Context>
and change /context to the name of your application’s context. Now when there is a change in the application Eclipse will signal a redeploy, but Tomcat will ignore it. If you change an area of code that can’t be hot-swapped then Eclipse will tell you (as normal), but at least now you won’t have to restart your container all the time.

Serializing and Deserializing custom objects using JiBX

Posted by on 16 Dec 2008 in Techie
JiBX is a framework for binding XML data to Java objects (I stole that from their website!). There are a standard set of converters in the JiBX library that will help you convert your own classes to XML. There is a bit more information in the conversions section of their website. Converters for all the privative types and some of the standard objects are supported in the class org.jibx.runtime.Utility, but some of the newer classes are not. One set of methods which is missing is a serializer and deserializer to convert java.util.UUID. Let’s say you want to serialize the following class: 
package uk.co.bigsoft.myproj;
import java.util.UUID ;

public class MyClass
  {
  private UUID ident;
  MyClass()
    {
    //
    }
  public UUID getIdent()
    {
    return ident;
    }
  public void setIdent(UUID u)
    {
    ident= u;
    }
  }
You must compile the following JiBX binding: 
<?xml version="1.0" encoding="UTF-8"?>
<binding>
  <format type="java.util.UUID" serializer="uk.co.bigsoft.formatters.Formatter.fromUUID" deserializer="uk.co.bigsoft.Formatter.formatters.toUUID"/>

  <mapping name="myclass" class="package uk.co.bigsoft.myproj.MyClass">
    <value name="ident" field="ident"/>
  </mapping>
</binding>
JiBX will take the binding and start converting the value mappings. If it recognises the type of the attribute you are trying to convert it will use its own converter, but in this case it will see that MyClass.ident is a UUID and use the user defined one. 
package uk.co.bigsoft.formatters;
import java.util.UUID ;

public class Formatter
  {
  public static UUID toUUID (String u)
    {
    return UUID.fromUUID(u);
    }
  public static String fromDate (UUID u)
    {
    return u.toString() ;
    }
  }
The serializer and deserializer’s names are defined like this: fromType and toType, but they can be called anything. The only restriction is that they must be static.